The theory underlying tissue engineering applications essentially consists in combining a natural or synthetic matrix with cells from specific tissue source, in such a way that said cells may be grown in a laboratory and then transplanted into a human body.
Moreover, by using implant materials able to positively interact with cells, specific tissue regeneration processes can be initiated and the de novo formation of whole tissue structures achieved. In many instances, such regenerated tissues will be capable of performing their normal functions eventually lost because of previously suffered damages.
Silk fibroin is a bio-material that can be used in surgery as implant material as well as for tissue engineering applications. In this context it is proper to remark that silk fibroin has very good properties such high strength coupled with flexibility, blood compatibility, water permeability and permeability to oxygen; all this makes silk fibroin an excellent candidate for biomedical applications as either in the form of non-woven membranes and fibers or in the form of woven membranes and fibers.
Silk fibroin makes up 75% to 80% by weight of the raw silk. The silk content of the protein sericin, which surrounds the two kinds of fibroin filaments, varies from 20% to 25% by weight depending on the species, origin, and culture conditions of the raw silk.
Silk fibroin is a fibrous protein whose hierarchical structure consists of fibrils and micro-fibrils, in which the fibers are arranged in a highly oriented crystalline form.
Silk fibroin easily dissolves in a saturated aqueous solution of inorganic salts.
By desalting such a solution by dialysis, an aqueous solution of silk fibroin could be obtained.
Due to the alfa-elix structure of fibroin, these solutions are not stable, and the structure of the regenerated membrane that is obtained from these solutions can be easily changed by polar solvents, ageing, and physical forces (shear, vibration, mixing etc.).
In order to obtain silk fibroin, it is possible to use silk in the form of cocoons, silk textiles and waste silk.
The raw silk filament is not soluble in formic acid due to the presence of an external sericin layer.
In order to eliminate the sericin layer, silk must be first degummed.
With the term “degumming”, the partial or complete removal of the sericin that covers the two types of fibroin filaments is intended.
The degumming agents commonly used are mainly alkali-free soaps.
According to a degumming method known in the art, silk is treated in a soap bath at 95-98° C. for a period of 2-4 hours, depending on the quality and type of the fabric.
The use of silk fibroin as a cell culture matrix is already known (see for example the Italian Patent Application No. VR99A000082), as a burn wound dressing membrane (N. Minoura, M. Tsukada, M. Nagura, “Physico-chemical properties of silk fibroin membrane as a biomaterial” Biomaterials, 11, 430-434, 1990), as an enzyme-immobilization material (M. Demura, T. Asakura, T. Kuroo, “Immobilization of biocatalyst with Bombix mori silk fibroin by several kinds of physical treatment and its application to glucose sensors”, Biosensors, 361-372, 1989), and an oral dosage form ù(T. Hanawa, A. Wanabe, T. Tsuchiya, R. Ikoma, M. Hidika, M. Sugihara, “New oral dosage form for elderly patients: Preparation and characterization of silk fibroin gel”, Chem. Pharm. Bull. 43, 284-288, 1995).
However, silk fibroin membranes are very brittle and their preparation is difficult and time-consuming.
The use of textile methods would theoretically be possible to weave using merely degummed silk fibroin fibers in order to obtain a flexible fabric.
However, preparing three dimensional structures appears to be remarkably difficult, and the adaptability of a tissue obtained in such a way would not be entirely proper to the different applicative requirements in terms of specifically required mechanical attributes, structure, and even ability to interact with other cells.